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Computed Tomography (CT or CAT) Scan of the Kidney

Computed Tomography (CT or CAT) Scan of the Kidney

(Renal CT Scan)

Procedure overview

What is a CT or CAT scan of the kidney?

Computed tomography (CT or CAT scan) is a noninvasive diagnostic imaging procedure that uses a combination of X-rays and computer technology to produce horizontal, or axial, images (often called slices) of the body. A CT scan shows detailed images of any part of the body, including the bones, muscles, fat, and organs. CT scans are more detailed than standard X-rays.

In standard X-rays, a beam of energy is aimed at the body part being studied. A plate behind the body part captures the variations of the energy beam after it passes through skin, bone, muscle, and other tissue. While much information can be obtained from a standard X-ray, a lot of detail about internal organs and other structures is not available.

In computed tomography, the X-ray beam moves in a circle around the body. This allows many different views of the same organ or structure. The X-ray information is sent to a computer that interprets the X-ray data and displays it in a two-dimensional (2D) form on a monitor.

CT scans may be done with or without "contrast." Contrast refers to a substance taken by mouth or injected into an intravenous (IV) line that causes the particular organ or tissue under study to be seen more clearly. Contrast examinations may require you to fast for a certain period of time before the procedure. Your doctor will notify you of this prior to the procedure.

CT scans of the kidneys can provide more detailed information about the kidneys than standard kidney, ureter, and bladder (KUB) X-rays, thus providing more information related to injuries and/or diseases of the kidneys. CT scans of the kidneys are useful in the examination of one or both of the kidneys to detect conditions such as tumors or other lesions, obstructive conditions, such as kidney stones, congenital anomalies, polycystic kidney disease, accumulation of fluid around the kidneys, and the location of abscesses.

Other related procedures that may be used to diagnose kidney problems include KUB X-rays, kidney biopsy, kidney scan, kidney ultrasound, renal angiogram, and renal venogram. Please see these procedures for additional information.

Illustration of the anatomy of the kidney
Click Image to Enlarge

How do the kidneys work?

The body takes nutrients from food and converts them to energy. After the body has taken the food that it needs, waste products are left behind in the bowel and in the blood.

The kidneys and urinary system keep chemicals, such as potassium and sodium, and water in balance, and remove a type of waste, called urea, from the blood. Urea is produced when foods containing protein, such as meat, poultry, and certain vegetables, are broken down in the body. Urea is carried in the bloodstream to the kidneys.

Two kidneys, a pair of purplish-brown organs, are located below the ribs toward the middle of the back. Their function is to:

  • Remove liquid waste from the blood in the form of urine

  • Keep a stable balance of salts and other substances in the blood

  • Produce erythropoietin, a hormone that aids the formation of red blood cells.

  • Regulate blood pressure

The kidneys remove urea from the blood through tiny filtering units called nephrons. Each nephron consists of a ball formed of small blood capillaries, called a glomerulus, and a small tube called a renal tubule.

Urea, together with water and other waste substances, forms the urine as it passes through the nephrons and down the renal tubules of the kidney.

Reasons for the procedure

A CT scan of the kidney may be performed to assess the kidneys for tumors and other lesions, obstructions such as kidney stones, abscesses, polycystic kidney disease, and congenital anomalies, particularly when another type of examination, such as X-rays or physical examination, is not conclusive. CT scans of the kidney may be used to evaluate the retroperitoneum (the back portion of the abdomen behind the peritoneal membrane). CT scans of the kidney may be used to assist in needle placement in kidney biopsies.

After the removal of a kidney, CT scans may be used to locate abnormal masses in the empty space where the kidney once was. CT scans of the kidneys may be performed after kidney transplants to evaluate the size and location of the new kidney in relation to the bladder.

There may be other reasons for your doctor to recommend a CT scan of the kidney.

Risks of the procedure

You may want to ask your doctor about the amount of radiation used during the CT procedure and the risks related to your particular situation. It is a good idea to keep a record of your past history of radiation exposure, such as previous CT scans and other types of X-rays, so that you can inform your doctor. Risks associated with radiation exposure may be related to the cumulative number of X-ray examinations and/or treatments over a long period of time.

If you are pregnant or suspect that you may be pregnant, you should notify your doctor. Radiation exposure during pregnancy may lead to birth defects.

If contrast dye is used, there is a risk for allergic reaction to the dye. Patients who are allergic to or sensitive to medications should notify their doctor. You will need to let your doctor know if you have ever had a reaction to any contrast dye, and/or any kidney problems. A reported seafood allergy is not considered to be a contraindication for iodinated contrast.

Patients with kidney failure or other kidney problems should notify their doctor. In some cases, the contrast dye can cause kidney failure, especially in patients with underlying kidney problems or dehydration. Patients taking the diabetes medication metformin (Glucophage), or its derivatives, who receive contrast are at increased risk of developing a condition called metabolic acidosis, or an unsafe change in blood pH, and the drug may be halted for 48 hours after the procedure.

There is a small chance of contrast material leakage from the IV line (known as contrast extravasation), which may cause swelling, stinging pain, or skin damage at the IV site.

There may be other risks depending on your specific medical condition. Be sure to discuss any concerns with your doctor prior to the procedure.

Certain factors or conditions may interfere with the accuracy of a CT scan of the kidney. These factors include, but are not limited to, the following:

  • Metallic objects within the abdomen, such as surgical clips

  • Barium in the intestines from a recent barium study

  • Recent tests involving dye or other foreign substances

Before the procedure

  • Your doctor will explain the procedure to you and offer you the opportunity to ask any questions that you might have about the procedure.

  • If your procedure involves the use of contrast dye, you will be asked to sign a consent form that gives permission to do the procedure. Read the form carefully and ask questions if something is not clear.

  • Notify the technologist if you have ever had a reaction to any contrast dye, or if you are allergic to iodine. If you take metformin, you may be asked to discontinue it for at least 48 hours after your injection. 

  • Generally, there is no fasting requirement prior to a CT scan, unless a contrast dye is to be used. Your doctor will give you special instructions ahead of time if contrast is to be used and if you will need to withhold food and drink.

  • Notify the technologist if you are pregnant or suspect you may be pregnant.

  • Notify the technologist if you have any body piercing on your chest and/or abdomen.

  • Based on your medical condition, your doctor may request other specific preparation.

During the procedure

CT scanner

CT scans may be performed on an outpatient basis or as part of your stay in a hospital. Procedures may vary depending on your condition and your doctor's practices.

Generally, a CT scan of the kidney follows this process:

  1. You will be asked to remove any clothing, jewelry, or other objects that may interfere with the procedure.

  2. If you are asked to remove clothing, you will be given a gown to wear.

  3. If you are to have a procedure done with contrast, an intravenous (IV) line will be started in the hand or arm for injection of the contrast dye. For oral contrast, you will be given a liquid contrast preparation to swallow.

  4. You will lie on a scan table that slides into a large, circular opening of the scanning machine. Pillows and straps may be used to prevent movement during the procedure.

  5. The technologist will be in another room where the scanner controls are located. However, you will be in constant sight of the technologist through a window. Speakers inside the scanner will enable the technologist to communicate with and hear you. You may have a call button so that you can let the technologist know if you have any problems during the procedure. The technologist will be watching you at all times and will be in constant communication.

  6. As the scanner begins to rotate around you, X-rays will pass through the body for short amounts of time. You will hear clicking sounds, which are normal.

  7. The X-rays absorbed by the body's tissues will be detected by the scanner and transmitted to the computer. The computer will transform the information into an image to be interpreted by the radiologist.

  8. It will be important that you remain very still during the procedure. You may be asked to hold your breath at various times during the procedure.

  9. If the study is to be performed before and after the administration of IV contrast, you will be removed from the scanner after the first set of scans has been completed. A second set of scans will be taken after the contrast dye has been administered.

  10. If contrast dye is used for your procedure, you may feel some effects when the dye is injected into the IV line. These effects include a flushing sensation, a salty or metallic taste in the mouth, a brief headache, or nausea and/or vomiting. These effects usually last for a few moments.

  11. You should notify the technologist if you feel any breathing difficulties, sweating, numbness, or heart palpitations.

  12. When the procedure has been completed, you will be removed from the scanner.

  13. If an IV line was inserted for contrast administration, the line will be removed.

  14. You may be asked to wait for a short period of time while the radiologist examines the scans to make sure they are clear.

While the CT procedure itself causes no pain, having to lie still for the length of the procedure might cause some discomfort or pain, particularly in the case of a recent injury or invasive procedure, such as surgery. The technologist will use all possible comfort measures and complete the procedure as quickly as possible to minimize any discomfort or pain.

After the procedure

If contrast dye was used during your procedure, you may be monitored for a period of time for any side effects or reactions to the contrast dye, such as itching, swelling, rash, or difficulty breathing.

If you notice any pain, redness, and/or swelling at the IV site after you return home following your procedure, you should notify your doctor as this could indicate an infection or other type of reaction.

Otherwise, there is no special type of care required after a CT scan of the kidney. You may resume your usual diet and activities unless your doctor advises you differently.

Your doctor may give you additional or alternate instructions after the procedure, depending on your particular situation.

Online resources

The content provided here is for informational purposes only, and is not designed to diagnose or treat a health problem or disease, or replace the professional medical advice you receive from your doctor. Please consult your health care provider with any questions or concerns you may have regarding your condition.

This page contains links to other websites with information about this procedure and related health conditions. We hope you find these sites helpful, but please remember we do not control or endorse the information presented on these websites, nor do these sites endorse the information contained here.

American Cancer Society

American Urological Association

National Cancer Institute

National Institute of Diabetes and Digestive and Kidney Diseases

National Institutes of Health (NIH)

National Kidney Foundation

National Library of Medicine

Radiological Society of North America

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